In binary magnetic storage media, data are recorded in the form of reversals of magnetic flux transitions. The information content is in time separating any two flux reversals. A clocking reference is provided by a data separator to measure this time interval and hence to retrieve the data previously recorded. Due to speed variations of the media drive mechanism in a magnetic storage system, the data separator must operate in a closed loop feedback system, and the clock it provides must be capable of varying its frequency to track variations in speed of the recording medium. The clock is normally produced by a voltage controlled oscillator (VCO) whose frequency is controlled by a phase discriminator. The phase discriminator measures phase difference between the clock and the incoming data and causes the frequency of the VCO to vary accordingly. In this way a desired phase relationship between the two signals can be established to recover data pulses that are received. U.S. Pats. Nos. 3,614,635, 3,701,039, 4,034,309 and 4,682,121 disclose typical prior art systems of this general type.
These and other previously implemented phase discriminator and data separator systems known to applicants require excessive time to complete measurement of the phase difference between the clock and data denser than (2,7) run length limited (RLL) coded data. This causes bits to be dropped, race problems and incorrect synchronization when (1,7) or equivalent codes are employed.
The exact placement of a magnetic transition in time relative to its adjacent transitions can be altered by magnetic imperfection in the recording medium, interference from adjacent transitions, or noise and imperfection in the detection processes, as well as the variations in the media drive mechanism previously mentioned. All these influence readback errors. One way to recover errors encountered during readback is to offset the head with respect to the data track. When changing head offset, both the clock and the data are modified slightly to affect only those datum pulses that have just missed their respective detection windows. The head offset change may not always be in the direction to recover the errors. Moreover, changing head offset involves a mechanical movement of the head, which will degrade the performance.
Furthermore, in prior art systems known to applicants, the widths of the detected read datum pulse for address mark (AM) search operations were not standardized and resulted in data pulse widths being too wide or too narrow and therefore unrecognizable by the system. Whenever an AM search operation was called for, the data separators did not provide any re-synchronization and/or re-shaping of the readback data pulses. This could result in a missing AM due to improper recovery of the AM data bits.
There is a need for a phase discriminator and data separator system that includes a data shifting function (shifting earlier and/or later) for recovering error bits that have just missed their respective detection (time) windows and have been identified as potentially uncorrectable errors. In addition, it is desirable that in AM search operations, the readback pulses be standardized to one clock period (preferably derived from servo data) to eliminate the effects due to pulsewidth variations and delay skew between the output data and the clock.